For as long as electromagnetic radiation has been used for communications, there have been attempts to interfere with those communications by jamming the radio frequency transmissions. Consequently, there has been continuing interest in systems that defeat the jamming by extracting the useful components from the jammed signal.
For reasons such as cost, power constraints, and simplicity, RF jamming typically is performed on a narrow frequency band compared with the broad frequency band of the useful signal. However, separating the useful broadband signal from the narrowband jamming signal is difficult because the actual frequencies of the narrowband jammers generally are not known beforehand. In the past, adaptive filters using nonlinear feedback and various algorithms have been implemented with varying degrees of success in filtering the jammed signal. The primary drawback of such systems has been their complexity, with the attendant problems of high cost and low reliability. Thus, there is a need for a system that is relatively simple but effective in removing narrowband jamming noise from a broadband RF signal.
In the field of nonlinear optics, the photorefractive effect occurs in a class of crystals such as barium titanate (BaTiO.sub.3) and strontium barium niobate (SBN). A two-wave mixing phenomenon is known in which two coherent beams of light crossing within the volume of a photorefractive crystal can exchange energy. In such two-wave mixing, it is known that one beam can be amplified at the expense of the other with the direction of the energy flow determined by the orientation of the photorefractive crystal. A time constant associated with two-wave mixing in photorefractive crystals is approximately inversely proportional to the intensity of the light and ranges from milliseconds to seconds at commonly used light intensities. Furthermore, the two-wave mixing phenomenon is critically dependent on the relative coherence of the two beams of light. It is this coherence dependent two-wave mixing phenomenon that is utilized in the present invention to remove narrowband jamming noise from a broadband RF signal.